E. Azéma, F. Radjaï, R. Peyroux, G. Saussine, Physical Review E , 76, 011301 (2007).

A map of the contact network composed of very weak, intermediate, and strong contacts in the pentagon packing.

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics, except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit “strong” and “weak” force networks in the pentagon packing, but our simulation data also provide evidence for a large class of “very weak” forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zigzag aspect and a decreasing exponen- tial probability distribution as in a disk packing.

E. Azéma, F. Radjaï, R. Peyroux, F. Dubois, G. Saussine, Physical Review E, 74, 031302 (2006).

By means of two-dimensional contact dynamics simulations, we analyze the vibrational dynamics of a confined granular layer in response to harmonic forcing. We use irregular

The geometry of the packing.

polygonal grains allowing for strong variability of solid fraction. The system involves a jammed state separating passive loading and active unloading states. We show that an approximate expression of the packing resistance force as a function of the displacement of the free retaining wall from the jamming position provides a good description of the dynamics. We study in detail the scaling of displacements and velocities with loading parameters. In particular, we find that, for a wide range of frequencies, the data collapse by scaling the displacements with the inverse square of frequency, the inverse of the force amplitude, and the square of gravity. Interestingly, compaction occurs during the extension of the packing, followed by decompaction in the contraction phase. We show that the mean compaction rate increases linearly with frequency up to a characteristic frequency and then it declines in inverse proportion to frequency. The characteristic frequency is interpreted in terms of the time required for the relaxation of the packing through collective grain rearrangements between two equilibrium states.